Exemplary thermoelectric generators to which the improvement of this invention applies and the principles of operation thereof have been described in U.S. Pat. Nos. 3,458,356 and 4,094,877. "Sodium heat engine" is the name commonly given to such thermoelectric generators which electrochemically expand sodium metal across a solid electrolyte. While other alkali metals may be employed in the generator of this invention, the sodium heat engine is described herein as exemplary of such generators.
The sodium heat engine generally comprises (FIG. 1) a closed container separated into a first and second reaction zone by a solid electrolyte. Liquid sodium metal is present in the first reaction zone (i.e., on one side of the solid electrolyte) and the first reaction zone is maintained during operation of the device at a pressure higher than that of the second reaction zone. In the lower pressure second reaction zone, a permeable, electrically conducting electrode is in contact with the solid electrolyte. During operation of such a device, a heat source raises the temperature of liquid sodium metal within the first reaction zone to a high temperature (T.sub.2) and a corresponding high vapor pressure (P.sub.2) which creates a sodium vapor pressure differential across the solid electrolyte. In response to this pressure differential, the elemental sodium gives up electrons to an electrode in contact with the sodium metal and the resulting sodium ions migrate through the solid electrolyte. The electrons having passed through an external load, neutralized sodium cations at the permeable electrode-solid electrolyte interface. Elemental sodium metal evaporates from the permeable electrode and migrates through the low pressure (P.sub.1) second reaction zone (i.e., vacuum space) to a low temperature (T.sub.1) condenser. The condensed liquid sodium may then be returned back to the higher temperature region within the first reaction zone, e.g., by means of a return line and an electromagnetic pump, to complete a closed cycle. Thus, during operation of the device, sodium passes from the first reaction zone to the second and, where the device includes means for pumping the sodium back to the first reaction zone, the sodium completes the cycle. The process occurring in the electrolyte and at the sodium electrolyte and electrode-electrolyte interfaces is nearly equivalent to an isothermal expansion of the alkali metal from pressure P.sub.2 to P.sub.1 at the temperature T.sub.2. No mechanical parts need move, and the work output of the process is electrical only.
In continuous operation, the sodium heat engine requires a return line to bring the condensed sodium from the second reaction zone to the interior of the first reaction zone. The sodium in the return line must be kept above its melting point to prevent plugging of the line. Preferably, the temperature of the returning liquid sodium is sufficiently high to prevent the electrolyte from being thermally shocked by the sodium as it enters the hot first reaction zone. In previous designs, the return line has been brought to the first reaction zone by a route external to the condenser chamber as depicted in FIG. 1 of this application or FIG. 1 of U.S. Pat. No. 4,098,958. Such routing has required special measures to ensure that the temperature of the sodium within the return line is properly maintained. Heating tapes, strapping of the tubing to the exterior of the condenser, etc., have been used for this purpose. By means of this invention, heat energy present within the second reaction zone can be efficiently employed for heating the returning liquid alkali metal.